1. Field of the Invention
This invention generally pertains to the field of Portland cement manufacturing and more particularly, to the combustion process in Portland cement manufacturing.
2. Description of Related Art
The Portland cement manufacturing process has, in its most general form, long been established. As shown in FIG. 1, a raw feed material 12 is fed into a cement kiln system 10 wherein it is heated until it calcines and transforms into a material 13 known as xe2x80x9ccement clinkerxe2x80x9d or xe2x80x9cclinkerxe2x80x9d. More particularly, the raw feed material 12 is fed into the cyclone preheater 14. The preheater 14 heats the raw feed 12 to a temperature ready for calcination and then passes the preheated feed 12 to the precalciner 16. The precalcined feed 12 then enters the rotary kiln 18 wherein it is transformed into the clinker 13 that is deposited in clinker cooler 20.
The calcination reaction in the precalciner 16 takes place at a relatively narrow temperature range of about 1500 to 1600xc2x0 F. The heat for calcination is provided by the flameless combustion of fuel injected at point 17 of the precalciner 16 and the preheated feed 12 can reach over 90% calcination before entering the rotary kiln 18. The residual carbonate is calcined in the rotary kiln 18, where the temperature of the calcined feed 12 is raised to a clinkering temperature of about 2650xc2x0 F. by firing a mixture of fuel and air by kiln burner 22. At the clinkering temperature, approximately 25% of the hot meal is liquefied and, to reach the clinkering temperature, a flame temperature of over 3500xc2x0 F. is required.
The tumbling action of the rotary kiln forms the partially liquefied feed into clinker nodules 13 that drop into the clinker cooler 20, where they cool and are taken for pulverization. The air in the clinker cooler 20 cools the clinker 13 by absorbing heat therefrom and the air is thereby heated. This heated air is recycled into the rotary kiln 18 as xe2x80x9csecondary airxe2x80x9d to support the combustion therein and into the precalciner 16 via a tertiary air duct 21 as xe2x80x9ctertiary airxe2x80x9d.
In the late 1950""s and 1960""s, prior to the development of the precalciner, the industry began experimenting with oxygen enrichment of kiln combustion as a potential refinement. Martin J. La Velle, in 1959, suggested such in xe2x80x9cOxygen Enrichment of Primary Air Can Improve Kiln Production,xe2x80x9d published in the journal Rock Products, but no practical application is known. Oxygen enrichment of secondary air has been practiced on several occasions. A report by Robert A. Gaydos, entitled xe2x80x9cOxygen Enrichment of Combustion Air in Rotary Kiln,xe2x80x9d published by the Portland Cement Association, indicated that oxygen enrichment could improve kiln production.
One common method of oxygen enrichment for a rotary kiln is to place an oxygen lance in between kiln burner 22 and the feed 12 in the rotary kiln 18. Oxygen is injected through the lance of the burner 22 at a certain tip velocity. Since the underside of the flame in contact with pure oxygen will exhibit high flame temperature, the oxygen lance is placed so that excessive flame temperature will not impact refractory and kiln coating. The drawbacks of this practice are many: (1) oxygen lance is subject to high temperature, (2) proper direction and velocity of oxygen jet is critical, (3) high flame temperature promotes increased NOx formation, and (4) high flame temperature may adversely impact refractory life.
The precalciner 16 also contains a firing point, which firing point can be equipped with multiple burners. The preheated feed 12 entering the precalciner 16 is suspended and calcined in the vessel thereof. The fuel injected at point 17 supplies the heat necessary to dissociate carbon dioxide from limestone in the feed 12. The combustion air is primarily supplied as tertiary air from the clinker cooler 20 through the tertiary air duct 21. Due to the large amount of limestone powder present, the feed 12 undergoes an endothermic reaction in the precalciner vessel and the combustion is flameless. Oxygen enrichment in the precalciner has a reduced risk to refractory life or to increased NOx formation, mainly due to the low temperature combustion.
However, a major deterrent to oxygen enrichment in cement kiln systems has always been the cost of oxygen. Some reports have also indicated technical concerns arising from s oxygen enrichment in the kiln, such as refractory life, burning zone shift and coating stability. Thus, despite the optimistic tenor of some experiment reports and even though it is commonly used in lime kilns, oxygen enrichment in cement kiln systems never became a common practice because of a variety of technological and economic considerations.
By contrast, so far as is known, nobody has attempted oxygen enrichment of combustion in the precalciner. One authority in the field, Kurt E. Peray in The Rotary Kiln (2nd ed. 1985), has suggested this specifically for precalciners without tertiary air ducts or for xe2x80x9cair throughxe2x80x9d systems in order to reduce the excess air drawn through the kiln, but admits not knowing of any previous attempts and makes no mention of oxygen enrichment in systems using a tertiary air duct. Further, Peray admits that xe2x80x9c[t]his idea would require research before deciding whether or not it could be feasibly implementedxe2x80x9d. Id., at p. 75. However, after approximately 12 years, nobody in the industry has reported such an experiment and certainly there are no known successful attempts.
Thus, it is desirable to develop a cement manufacturing process with higher productivity than is otherwise available from existing pyroprocessing equipment. It would also be desirable to avoid the disadvantages of traditional introduction of oxygen to the kiln burner.
The invention in its various embodiments includes a method and an apparatus for improving combustion in a cement kiln system. The method in one embodiment comprises enriching the tertiary air stream to the precalciner of a cement kiln system with oxygen. The apparatus in one embodiment comprises a precalciner and an oxygen source coupled to the precalciner.